linux-kernel - Re: [PATCH v5 15/16] ima: Move dentries into ima

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Date:   Fri, 10 Dec 2021 10:26:50 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     jejb@...ux.ibm.com, Stefan Berger <stefanb@...ux.ibm.com>,
        Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-integrity@...r.kernel.org, serge@...lyn.com,
        containers@...ts.linux.dev, dmitry.kasatkin@...il.com,
        ebiederm@...ssion.com, krzysztof.struczynski@...wei.com,
        roberto.sassu@...wei.com, mpeters@...hat.com, lhinds@...hat.com,
        lsturman@...hat.com, puiterwi@...hat.com, jamjoom@...ibm.com,
        linux-kernel@...r.kernel.org, paul@...l-moore.com, rgb@...hat.com,
        linux-security-module@...r.kernel.org, jmorris@...ei.org
Subject: Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

On Fri, 2021-12-10 at 09:26 -0500, James Bottomley wrote:
> On Fri, 2021-12-10 at 09:17 -0500, Stefan Berger wrote:
> > On 12/10/21 08:02, Mimi Zohar wrote:
> > > On Fri, 2021-12-10 at 07:40 -0500, Stefan Berger wrote:
> > > > On 12/10/21 07:09, Mimi Zohar wrote:
> > > > > On Fri, 2021-12-10 at 12:49 +0100, Christian Brauner wrote:
> > > > > > > There's still the problem that if you write the policy,
> > > > > > > making the file disappear then unmount and remount
> > > > > > > securityfs it will come back.  My guess for fixing this is
> > > > > > > that we only stash the policy file reference,
> > > > > > > create it if NULL but then set the pointer to PTR_ERR(-
> > > > > > > EINVAL) or something and refuse to create it for that
> > > > > > > value.
> > > > > > Some sort of indicator that gets stashed in struct ima_ns
> > > > > > that the file does not get recreated on consecutive mounts.
> > > > > > That shouldn't be hard to fix.
> > > > > The policy file disappearing is for backwards compatibility,
> > > > > prior to being able to extend the custom policy.  For embedded
> > > > > usecases, allowing the policy to be written exactly once might
> > > > > makes sense.  Do we really want/need to continue to support
> > > > > removing the policy in namespaces?
> > > > I don't have an answer but should the behavior for the same
> > > > #define in this case be different for host and namespaces? Or
> > > > should we just 'select IMA_WRITE_POLICY and IMA_READ_POLICY' when
> > > > IMA_NS is selected?
> > > The latter option sounds good.  Being able to analyze the namespace
> > > policy is really important.
> > 
> > Ok, I will adjust the Kconfig for this then. This then warrants the 
> > question whether to move the dentry into the ima_namespace. The
> > current code looks like this.
> > 
> > #if !defined(CONFIG_IMA_WRITE_POLICY) &&
> > !defined(CONFIG_IMA_READ_POLICY)
> >          securityfs_remove(ns->policy_dentry);
> >          ns->policy_dentry = NULL;
> >          ns->policy_dentry_removed = true;
> > #elif defined(CONFIG_IMA_WRITE_POLICY)
> > 
> > With IMA_NS selecting IMA_WRITE_POLICY and IMA_READ_POLICY the above 
> > wouldn't be necessary anymore but I find it 'cleaner' to still have
> > the dentry isolated rather than it being a global static as it was
> > before...
> 
> This is really, really why you don't want the semantics inside the
> namespace to differ from those outside, because it creates confusion
> for the people reading the code, especially with magically forced
> config options like this.  I'd strongly suggest you either keep the
> semantic in the namespace or eliminate it entirely.
> 
> If you really, really have to make the namespace behave differently,
> then use global variables and put a big comment on that code saying it
> can never be reached once CONFIG_IMA_NS is enabled.

The problem seems to be with removing the securityfs policy file. 
Instead of removing it, just make it inacessible for the "if
!defined(CONFIG_IMA_WRITE_POLICY) && !defined(CONFIG_IMA_READ_POLICY)"
case.

thanks,

Mimi